Flame-resistant fiber is so excellent in heat resistance and flame retardance as to be widely utilized, for example, for spatter sheets for protecting the human body from high-heat iron powder and weld sparks which scatter in welding operations, and fire-resistant heat insulators for aircraft, leading to an increasing demand in those fields.
Flame-resistant fiber is important also as intermediate raw materials for obtaining carbon fiber. The carbon fiber has such mechanical, chemical properties and lightweight properties as to be widely used for various uses, for example, materials for aviation and space such as aircraft, rockets, sporting goods such as tennis rackets, golf shafts and fishing rods, and in the field of transport machines such as vessels and automobiles. In recent years, carbon fiber has such a high electrical conductivity and heat radiation as to be strongly required for application to electronic equipment parts such as portable telephones and personal computer cabinets, and electrodes of fuel cells.
Carbon fiber is generally obtained by a method of carbonizing flame-resistant fiber by heating at high temperature in an inert gas such as nitrogen. With regard to flame-resistant fiber, for example, polyacrylonitrile (PAN)-based flame-resistant fiber is obtained by making PAN-based precursor fiber flame-resistant (cyclization reaction and oxidation reaction of PAN) at a high temperature of 200 to 300° C. in air.
However, this reaction for making flame-resistant is an exothermic reaction and a reaction in a fibrous form, namely, a solid-phase state. Therefore, a long treatment time is required for temperature control, and the degree of fineness of PAN-based precursor fiber needs to be limited to a fine size below a specific value so that it is flame-resistant within a desired time. Thus, the presently known process of achieving making flame resistance is regarded as difficult and not a sufficiently efficient process.
With regard to flame-resistant products, it is substantially difficult to obtain flame-resistant formed products except for fibers, such as planar shapes of sheets and films and various cubic shapes, due to the difficulty of heat removal for the reason that the reaction for achieving flame resistance is an exothermic reaction as described above. Accordingly, flame-resistant formed products are limited to fiberform products, and in the present circumstances, planar sheets are manufactured by making such fiberform products into fabrics.
When flame-resistant fiber with an optional degree of fineness and flame-resistant products except fiberform products (flame-resistant formed products), such as sheet-like products and cubic molded products are obtained, the use of flame-resistant formed products is markedly extended. In addition, the appropriateness of manufacturing conditions and carbonizing conditions thereof allows carbon fiber with an optional degree of fineness and carbon products except fiberform products (carbon product) such as a carbon product group of sheet-like carbon and cubic carbon molded products. The improvement of yield while maintaining high physical properties of carbon products brings advantages in costs.
Dissolution by solvents has been studied as a method for solving the above technical problem.
For example, a technique is disclosed such that acrylonitrile polymer powder is heated in an inert atmosphere until the density becomes 1.20 g/cm3 or more, and thereafter dissolved in solvent and fiberized into a fibriform product, which is heat-treated (for example, refer to Japanese Patent Publication No. 63-14093B).
However, the problem is that viscosity change of the solution over time is so great as to frequently cause thread breakage by reason of using acrylonitrile polymer powder with low flame resistance. A device made of special materials and having corrosion resistance is to be used with strongly acidic solvents such as sulfuric acid and nitric acid for easily decomposing general organic polymers, but is impractical due to costs.
A method is proposed such that heat-treated acrylonitrile polymer powder and not heat-treated acrylonitrile polymer powder are mixed and similarly dissolved in acidic solvent (for example, refer to Japanese Patent Publication No. 62-57723B). However, the problem is still not solved on allowing corrosion resistance to a device as described above and instability of solution.
In addition, the conversion of polyacrylonitrile to a polymer having a cyclized chemical structure by heat-treating dimethylformamide solution of polyacrylonitrile is disclosed (for example, refer to “Polymer Science (USSR),” 1968, Vol. 10, page 1537). However, the polymer solution is a dilute concentration of 0.5% and so low in viscosity as to be substantially difficult in forming into fibers, and a rise in concentration thereof causes the polymer to be deposited and incapable of being used as a solution.
On the other hand, a solution in which polyacrylonitrile is denatured with a primary amine is disclosed (for example, refer to “Journal of Polymer Science, Part A: Polymer Chemistry,” 1990, Vol. 28, page 1623). However, the solution is such as to impart a hydrophilic property to polyacrylonitrile itself with low flame resistance, and totally differs in the technical idea from a flame-resistant polymer-containing solution.
A technique is disclosed such that the yield can be improved together with high physical properties from flame-resistant fiber to carbon fiber under special carbonizing conditions (for example, refer to Japanese Patent Publication No. 26365093B). However, compatibility therebetween in an easier method has been demanded.
In view of the above-mentioned problems, it could be helpful to provide a flame-resistant polymer so excellent in forming processability as to produce a flame-resistant formed product also in unprecedented shapes, a flame-resistant polymer-containing solution and a manufacturing method for conveniently producing these, and to further provide a flame-resistant formed product, a carbon molded product employing the flame-resistant polymer and a manufacturing method for conveniently producing them.